A thermal flow meter that measure a flow rate of gas is configured to include an air flow sensing portion for measuring a flow rate, such that a flow rate of the gas is measured by performing heat transfer between the air flow sensing portion and the gas as a measurement target. The flow rate measured by the thermal flow meter is widely used as an important control parameter for various devices. The thermal flow meter is characterized in that a flow rate of gas such as a mass flow rate can be measured with relatively high accuracy, compared to other types of flow meters.
However, it is desirable to further improve the measurement accuracy of the gas flow rate. For example, in a vehicle where an internal combustion engine is mounted, demands for fuel saving or exhaust gas purification are high. In order to satisfy such demands, it is desirable to measure the intake air amount which is a main parameter of the internal combustion engine with high accuracy. The thermal flow meter that measures the intake air amount guided to the internal combustion engine has a bypass passage that takes a part of the intake air amount and an air flow sensing portion arranged in the bypass passage. The air flow sensing portion measures a state of the measurement target gas flowing through the bypass passage by performing heat transfer with the measurement target gas and outputs an electric signal representing the intake air amount guided to the internal combustion engine. This technique is discussed, for example, in JP 2011-252796 A (PTL 1).
In order to measure a flow rate of a gas with high accuracy using a thermal flow meter, it is necessary to position and fix an air flow sensing portion of the thermal flow meter in the bypass passage provided in the thermal flow meter co receive a gas flowing through the main passage with high accuracy. In the technique discussed in PTL 1, a casing having the bypass passage having a hole formed to insert the air flow sensing portion is formed of resin in advance, and a sensor assembly having the air flow sensing portion is formed separately from the casing, so that the sensor assembly is fixed to the casing while the air flow sensing portion is inserted into the hole of the bypass passage. An elastic adhesive is filled in a gap between the hole of the bypass passage and she air flow sensing portion and a gap of the portion where the sensor assembly is inserted into the casing, so that an elastic force of the adhesive absorbs a linear expansion difference therebetween.
In such a structure, it is difficult to accurately set and fix a positional relationship or an angle relationship between the air flow sensing portion and the bypass passage when the sensor assembly is inserted into the casing. That is, a positional relationship or an angle relationship between the sensor assembly and the bypass passage provided in the casing may easily change depending on a condition of the adhesive. For this reason, in the thermal flow meter of the related art, it is difficult to further improve the detection accuracy of the flow rate. In general, the thermal flow meter is produced in large quantities. In this large-quantity production process, when the air flow sensing portion is fixed to the bypass passage using an adhesive with a predefined positional relationship or an angular relationship, it was difficult to define a positional relationship or an angular relationship between the air flow sensing portion and the bypass passage during bonding of the adhesive and in a solidification process of the adhesive and holding such as positional relationship with high accuracy. For this reason, it was difficult to further improve measurement accuracy of the thermal flow meter in the related art.
In addition, in the technology disclosed in PTL 1, when the air flow sensing portion is disposed in the bypass passage, the end portion of the air flow sensing portion is exposed in the bypass passage, and the measurement target gas flowing through the bypass passage conflicts with the end portion of the air flow sensing portion to generate a vortex of the measurement target gas (also referred to as a wing tip vortex). The vortex generated in the end portion of the air flow sensing portion is induced to the downstream side by the measurement target gas flowing through the bypass passage, and reaches the heat transfer surface of the air flow sensing portion depending on a position of the heat transfer surface of the air flow sensing portion, and the measurement accuracy of the thermal flow meter is degraded. Therefore, in the related art, it is difficult to further improve the measurement accuracy of the thermal flow meter.
Regarding the above problem, for example, JP 2003-502682 W (PTL 2) discloses a technology of suppressing underflow (Unterstroemung) by forming a transition portion between the outer side surface of a sensor supporting body and the edge surface of the bypass passage to be flush with each other.
In she technology disclosed in PTL 2, a seal means is disposed between the end surface of the sensor supporting body and the edge surface of the bypass passage to fill a gap formed due to a manufacturing allowable error, so that the transition portion is formed between the outer side surface of the sensor supporting body and the edge surface of the bypass passage to be flush with each other. Alternatively, the end surface side of the sensor supporting body is inserted, into a notch provided in the edge surface of the bypass passage, a partitioning wall provided with a cover to close the bypass passage is engaged in the notch, and the seal means is disposed between the edge surface side of the partitioning wall and the outside of the sensor supporting body facing the cover to fill a gap formed due to a manufacturing allowable error.